1. Field of the Invention
This invention relates to an engine driven arc welder or welding machine in which the external output characteristic is caused to have constant current characteristic.
2. Description of the Prior Art
Hitherto, in welding work by engine driven arc welders used in outdoor construction works, manual welding is the main current. In view of this, there has been conventionally proposed a system in which power supply of constant current characteristic having high stability of arc is applied to the outdoor engine driven arc welder. The circuit configuration of the conventional system is shown in FIG. 10. In the system of FIG. 10, a chopper circuit for providing general constant current characteristic is constituted. Namely, an output from generator 32 connected to engine 31 is rectified by rectifier 33 and is then smoothed by smoothing capacitor 34 so that it is caused to be power having lower impedance even when high speed operation of transistor is carried out. Switching transistor 35 carries out ON/OFF operation of power thus obtained at higher frequency and by large current. Reactor 36 suppresses sudden increase or decrease of current and smooths output caused to undergo switching by switching transistor 35, thus to stabilize the welding condition. In FIG. 10, reference numerals 37, 38 denote welding terminals, reference numeral 39 denotes a current detector, reference numeral 40 denotes a constant current control section, and reference numeral 41 denotes a transistor drive section for driving switching transistor 35.
However, with such welder of constant current characteristics, in the case where high cellulose covered or coated welding rod for which large short-circuit current is required is used, there was the problem that deposition is apt to take place, giving rise to welding defect.
As a method of increasing current at the time of short-circuit of welding, the apparatus described in Jikkohei No. 4-35007 (Japanese Utility Model Publication No. 35007/1992) has been proposed. In accordance with the apparatus above-mentioned, within current control circuit using thyristor, a detected value of arc current detecting circuit is delivered to current setting circuit to carry out adjustment of detection level to compare the detected value with a reference value at firing circuit via output characteristic switching circuit in order to carry out phase control. The technology of the apparatus contemplates suddenly increasing current at the time of short-circuit. Further, there is proposed, in the Tokkaihei No. 5-185226 (Japanese Patent Application Laid Open No. 185226/1993), the technology in which direct current (d.c.) in d.c. arc welder is converted into high frequency alternate current (a.c.) to carry out constant current welding by means of an output transformer for controlling the high frequency a.c. so that it becomes equal to an output voltage necessary for welding and output side rectifier of diode bridge configuration for rectifying this output to set a welding current to switch over characteristics by the operation of changeover switch based on lowering of voltage due to occurrence of short-circuit during welding.
In the case of the above-mentioned high cellulose system covered welding rod used outdoor, since welding is carried out in the state where the arc length is caused to be very short, there is no complete short-circuited state unless the welder has drooping characteristic. When current is suddenly increased resulting from the voltage being lowered a uniform welding state cannot be provided. This current fluctuation is insufficient.
Particularly, in the engine driven arc welder, unlike the above-mentioned d.c. arc welder, high frequency a.c. inverter, output transformer of high frequency a.c., and the like are not required.
In addition, in the case of the high cellulose system covered welder used in Europe or U.S.A., the above-mentioned problems were further conspicuous.
Moreover, when constant current output characteristic having high arc stability is employed in the engine driven arc welder, in the case where voltage drop of welding cable becomes large in dependency upon choice of operator in the vicinity where maximum current output is set, or in the case where long arc length is adopted in dependency upon kind of covered rods, welding output voltage is increased. For this reason, a generator and semiconductor are used having a capacity capable of providing full output of the maximum output curve in the constant current output characteristic and an engine of large output capable of driving such generator. Alternatively, there is employed the technology described in the Jikkohei No. 3-49802 (Japanese Utility Model Publication No. 49802/1991) as the technology in which slanting portion, which is the overload region of FIG. 6, is not caused to be outputted. In accordance with this technology, since the welder is connected to a commercial power supply, it is possible to prepare load characteristic in a stationary manner, wherein the power supply is also stable. Namely, this welder is designed on the premise of substantially the fixed condition.
However, in the case where the above-mentioned technology is utilized for an engine driven arc welder used at an outdoor location where no commercial power supply is installed, since its output characteristic is always set in a stationary manner, welding output would exceed required engine output unless an engine having margin in output is employed because of lowering of engine output due to change of weather condition, aged deterioration, etc. and unevenness of engine output. As a result, the engine rotation number would become extremely lower, so welding output becomes unstable. This might cause occurrence of welding defect, deterioration of the engine by overload of the engine, production of a great quantity of black smoke and lowering of the life time due to deterioration of cooling efficiency of welding generator.